Eat healthy, work out and try to get good sleep, if you can.
Working out makes your brain produce a bunch of fun stuff that makes you feel great.
Seratonin, dopamine...
Gain 'ilm starting from tawheed I repeat tawheed
Grow closer to the sunnah
Be sincere in all your actions
Make sure they are for the sake of Allah
'Ilm is what helps us fight the shaytaan and he is the one that wants you to be sad. So fight back by gaining ilm and sincerely putting it into practise fillah
Make du'a that Allah keeps your heart firm on the deen and you steadfast
Sorry to near about your situation. May Allah swt grant you shifa, Ameen. Recite often "Audhubillah min al shaitan rajeem (I seek Allah's protection/refuge/shelter from Shaitaan, the Accursed one." and Ayatul Kursi. Read certain surahs in Quran such as Yasin for protection.
Learn and remember the 99 names of Allah: Reflect on these beautiful names on daily basis, you will attain true tranquillity of the heart which you cannot get from anywhere else.
When you're sad, remember Allah is Ar-Raqeeb, Al-Aleem, Al-Wadud, Al-Nur
When you feel hopeless and giving up, remember Allah is Al-Mujeeb, Al-lateef, Al-Fattah, Al-Hayyiy
When you're frightened, remember Allah is Al-Hafidh, Al-Maani`, Al-Qawiyy
When you're struggling to survive, remember Allah is Ar-Razzaq, Al-Muhye
When you feel ashamed of your sins, remember Allah is At-Tawaab, Al-Haleem, As-Sitteer
When you're hurt, remember Allah is As-samad
When you have been separated from your loved one, remember Allah is Al-Jaami'
When people are ungrateful, remember Allah is Ash-shakoor
When no one helps you with your struggles, remember Allah is Al-Wakeel
When you're surrounded by lies and liars, remember Allah is Al-Haqq
When you're angry, remember Allah is Al-Basir, Al-Khabir, Al-Mudhill (The Humiliator)
When you are facing difficulties, remember Allah is Al-Kabeer
When you feel weak, remember Allah is Al-Azeez
When you keep making mistakes over and over again, remember Allah is As-sabur, Al-Gaffar, Ar-raheem
And most importantly, always remember Allah is As-Salaam....
For 99 human emotions you can feel, there are 99 beautiful names you can remember to find true tranquillity of the mind.
Alhamdulillah, Allah is truly Al-Wakeel.
Also Remember the stories of all the Prophets (pbut) that we know of - learn about what they went through and how patient and trusting of Allah, they were...make them your role models in life and when you are going through some hardships, remember how many Prophets pbut also went through harships and pain. Get strength and inspiration from their stories of hardship and difficulties with their beautiful characters and closeness to Allah swt. We have so much to learn from their stories and message in the Quran. May Allah swt increase us in good knowledge and practice, Ameen.
Read this beautiful series on Names of Allah swt which also includes many stories of Prophets (pbut):